On the morning of May 11, 1944, nearly 1,500 heavy bombers lifted off from bases in England, headed for targets in Nazi Germany. As the giant aircraft lumbered into formations, the wispy streaks of contrails marked their passage. Now, climate scientists are using weather data collected during those long ago combat missions to gain insight into how aircraft contrails can alter climate.

“Witnesses to the huge bombing formations recall that the sky was turned white by contrails,” says Rob MacKenzie of the University of Birmingham in United Kingdom, an author of the International Journal of Climatology study. But could the haze have changed England’s weather? It’s a question much on the minds of climate scientists these days, as they try to understand how jet contrails can alter cloud formation and atmospheric heat and energy patterns. So far, however, many of the studies are theoretical, and real-world case studies have been few and far between.

MacKenzie and his colleagues realized the Allied bombing raids offered an unusual “inadvertent environmental experiment.” First, they scoured archives for weather data collected during the campaign. In particular, the team looked for raids that involved over 1,000 aircraft and were followed by raid-free days with similar weather which might be used for comparison. Ultimately, the team found 56 candidate raids, and selected the 11th of May flight as their case study.

The data suggest contrails might make a difference. On the day of the raid, areas that the bombers flew over had more high cloud cover and temperatures that were 0.8 degrees Celsius cooler than bomber-free areas upwind. “One event cannot provide firm conclusions regarding the effect of contrails on climate,” the authors note. But the study shows that “historical data, such as WW2 bombing raids, may be an extremely important tool in closing the gap” theoretical and real-world studies of contrail impacts.

“This is tantalizing evidence that Second World War bombing raids can be used to help us understand processes affecting contemporary climate,” says MacKenzie. “By looking back at a time when aviation took place almost entirely in concentrated batches for military purposes, it is easier to separate the aircraft-induced factors from all the other things that affect climate.” – David Malakoff | July 11, 2011